This invention relates generally to self-emitting video display devices such as of the cathode ray tube (CRT) type and is particularly directed to a multi-grid electron gun such as used in a CRT having progressively reduced beam passing apertures in its charged grids in proceeding toward the electron gun""s cathode(s).
In a conventional electron gun such as used in either a monochrome or color CRT, energetic electrons are emitted from a cathode (or cathodes) and are directed to the gun""s beam forming region (BFR). The BFR includes the G1 control grid, the G2 screen grid and a portion of a G3 grid in facing relation with the G2 screen grid. The energetic electrons are directed through aligned apertures in these three grids and are thereby formed into a well-defined beam, or beams, having a very small, circular cross section. After transiting the electron gun""s BFR, the beams are directed through a focus lens, typically divided into a pre-focus lens and a main focus lens, for focusing the electron beams on a phosphor-bearing display screen of the CRT. The focus lens focuses each of the beams to a small spot on the CRT""s display screen, with the beams simultaneously deflected in a raster-like manner at very high speeds to form a video image on the display screen. In the case of a typical color CRT, three electron beams are simultaneously formed, focused, and converged to a single spot on the display screen. The three electron beams are then displaced in unison in a raster-like manner over the display screen in forming a color video image.
The beam passing apertures in the BFR are typically small in size, with the apertures in the electron gun""s G1 control grid and G2 screen grid typically on the order of 0.3 mm to 0.8 mm in diameter. The bottom portion of the G3 grid in facing relation with the G2 screen grid includes apertures which are somewhat larger in that they are typically on the order of 1 mm to 2 mm. The top portion of the G3 grid as well as the G4 and subsequent grids, including auxiliary dynamic modulation grids, have larger beam passing apertures which are typically on the order of 4.5 mm to 7.5 mm in diameter for color electron guns. Aperture size increases in proceeding toward the CRT""s display screen in the main focusing lens region in color electron guns due to the xe2x80x9ccommon lensxe2x80x9d design utilized in this portion of the electron gun. Even larger electron beam passing apertures are typically used in monochrome electron guns.
Up The electrons exiting the BFR are formed into a beam bundle for subsequent focusing by the pre-focus lens and main focus lens to a small spot on the CRT""s display screen. After exiting the electron gun""s BFR, the diameter of the beam increases continuously as the electrons travel in the direction of the display screen along the gun""s Z-axis. The electron beam expands in the R-direction which is transverse to the Z-axis. This electron beam expansion is due to the velocity of electrons along the R-direction, as well as to the space-charge effect in the beam caused by the mutual repulsion between the electrons in the beam.
The beam passing apertures in the various grids in an electron gun are generally of the same diameter. The primary reason for equal sized apertures in each of the gun""s charged grids relates to the use of a mandrel in electron gun assembly. A mandrel is inserted through each aligned array of beam passing apertures in the various grids to maintain the grids in common alignment during the beading process in electron gun assembly. The common sized beam passing apertures and the use of a generally cylindrical mandrel for grid alignment greatly simplifies and facilitates electron gun assembly.
As the electron beam expands in diameter after it exits the electron gun""s BFR, the focusing effect of each grid in the lens portion of the electron gun, where all of the grids have beam passing apertures of essentially the same size, becomes progressively stronger due to the progressively increasing diameter of the electron beam. Thus, the closer the charged grid is to the CRT""s display screen, the stronger is its focusing effect on the electron beam. Conversely, in the area of the BFR as well as in the lower portion of the gun""s pre-focus lens region, the charged grids have a reduced focusing effect on the electron beam due to the beam""s small diameter in this region. Because of the reduced focusing effect of the grids in this region, a larger dynamic focus voltage is required to correct for astigmatism of the deflected beam""s spot size caused by the CRT""s inline deflection yoke as well as to correct for out-of-focus effects which arise from the electron beam""s increased landing or throw distance. Reducing the dynamic focus voltage required to correct for astigmatism of the deflected beam places increased demands on electron gun design requirements.
The present invention addresses the aforementioned limitations of the prior art by providing progressively reduced electron beam passing aperture size in an electron gun for use in a CRT which increases electron beam focusing sensitivity without increasing beam spot aberration on the CRT""s display screen or the out-of-focus effects on the video image. By providing the BFR and pre-focus lens of the electron gun with progressively reduced electron beam passing aperture size in proceeding toward the gun""s cathode, increased electron beam focusing sensitivity is provided without increasing dynamic focus voltage or electron beam spot aberration on the display screen.
Accordingly, it is an object of the present invention to provide in an electron gun of a CRT increased electron beam focusing sensitivity for improved video image quality.
It is another object of the present invention to reduce the dynamic voltage is required in the electron gun of a CRT to correct for electron beam astigmatism and out-of-focus effects.
Yet another object of the present invention is to provide for the assembly of a multi-grid color CRT electron gun with charged grids having reduced diameter electron beam passing apertures using a mandrel.
The present invention contemplates an electron gun for use in a cathode ray tube (CRT) for producing a video image on a display screen, the electron gun comprising a cathode for providing energetic electrons; a beam forming region (BFR) aligned with the cathode and disposed intermediate the cathode and the display screen for receiving and forming the energetic electrons into an elongated, narrow beam, the BFR including plural spaced first charged grids each having one or more first aligned apertures, wherein the electrons are directed through the first aligned apertures and the electron beam increases in cross section in proceeding from the BFR toward the display screen; and an electrostatic lens disposed intermediate the BFR and the display screen and including plural spaced second charged grids each having one or more second aligned apertures through which the electron beam is directed for focusing the electron beam on the display screen, wherein the second aligned apertures decrease in size in proceeding in a direction from the display screen toward the BFR for increasing focusing sensitivity of the electrostatic lens on the electron beam.